What would happen if one attempted to run their projector without any triplet, using the field fresnel(And whatever focal length it has) alone?

Could this be viable in extra-long throw applications?

I know this sounds a little basic, but what exactly does the triplet DO? Magnification only? I could understand that, along with the zooming that you get when you make the image magnified (the beam) bigger by putting it further back in the beam. But how does the triplet interact with the field fresnel to focus the image?

With the standard lense kit, for example, the field fresnel is supposedly around ~320mm focal length, but the throw that that focal length produces is preserved even through the triplet.

I've just slowly realized that the interaction of the field fresnel and the triplet is one of the last things I don't understand in projection, anyone up for an optics lesson?

Huh?
Doesn't the field focus on a point several cm's away? If you double the focal point distance you would project a reversed image the size of the fresnel I guess, but it would scale in size really fast. How would a long throw theory work?

Long throw for the pro lense, short throw for the standard.

The field fresnel focuses the light from the LCD on a point 317mm(12.5") away. If the light continues undisturbed, it should achieve a throw distance of a little more than 1:1 (15"/12.48") diagonal:distance for a 15" LCD.

BTW, pics would probably help in an explanation here.

I don't know exactly what the triplet does, but I did try to remove it to see what would happen to the projected image. If I remember right, all I could see projected was a small piece of the center of the image.

Right, I understand this. At 31.7cm away the image is a pinpoint. At 63.4cm it should be roughly the size of the LCD and twice the size of the LCD at 95.1cm and so on. What I am unsure of is why you would remove the triplet entirely? A triplet is supposed to increase the throw to the screen while maintaining focus and image quality, right? Wouldn't removing it shorten the throw to the screen? Besides, all you are doing is, in essence, putting a fresnel in front of a really bright tv...
Maybe a diagram would help.

Yes please. <Brain?>

BTW, editing your original post and adding additional info is cheating

First off...

There can only be one focal point for a lens. An "infinte focus" is possible, but in the case of a projector, this is more or less unacceptable. The smallest detail that it then becomes possible to see is the aperature of the lens.

Take your fresnel outside and focus an image of the Sun on the ground. Melt some snow or something.

now move the fresnel towards or away from the ground. You don't get a larger image of the Sun, you get a blurry light. This will be the same result from your projector without a triplet.

In order to even give this a fighting shot, you'd need to place your 317mm focal length fresnel ~317mm or a bit further from the LCD, in effect using it as a projection lens. In this configuration, the absolute best that you could hope for is the same result as using a singlet lens as a projection lens, and that's only if the fresnel is kept optically flat, and it's an extremely well made fresnel. You would also need to be able to adjust that distance to get a projection at any distance.

V&J's earlier example of a pinpoint at 317mm, original LCD size at 634mm, etc is incorrect. There is no distance at which the LCD will properly focus to a pinpoint, with the fresnel anywhere near the LCD. The fresnel would need to be at a distance greater than it's own focal length from the LCD in order to focus at all, let alone to a pinpoint. For what it's worth, doing your melting snow (or frying stuff, if you're somewhere warm enough not to have snow, now) with sunlight doens't focus to a pinpoint, either. It focuses to a small image of the Sun at a magnification proportional to the focal length of the lens and the distance from that lens to the light source (In this case, the Sun.) There is an important difference...

Focusing happens when the SCATTERED rays from a point source are collected over the surface of the lens and redirected to a single point, and is a separate instance for each and every single point source withint your picture. In the case of a projector, each of those is considered to be an individual pixel. Only at the distance where the light from each pixel focuses apart from the light from a different pixel will an image be focused, and this is a very specific distance for a given lens. The only thing that can "focus" at any distance is a pinhole camera, and this is only the case because there is no lens effect.

Ideally, the field fresnel should have NOTHING to do with the projection lens optics, and in the case where the fresnels are together behind the LCD, this is more or less the case. The field fresnel's chief purpose in life is to collect as much light as possible and direct it towards the projection lens, acting as a light amplifier and not as a focusing lens. There are only 2 reasons for putting the field fresnel in front of the LCD. 1. To keep the light going through the LCD as straight as possible, and 2. To make use of the optical interaction for keystoning adjustment. It actually impedes the focusing of the triplet and results in (slightly) poorer image quality, which is acceptable because of the other benefits.

There is an optical interaction, which modifies the effctive focal length of the projection lens, proportional to the distance from the LCD. This is the case when any two lenses interact, such as when a person wears glasses. We can (and do) use this effect in a projector for keystoning, where we use the magnification effect of the lens to make a part of the screen seem larger to the projection lens. This is only possible within a fraction of the lens' focal elngth.

Well, enough on the essay for now... I'm sure half of the people looking at this stopped reading some time ago.

Yea...when I remove my triplet I see one big blob of white with hints of color on my screen...you need a triplet

thanks for all the details, i read it twice just to let it sink in a bit

i like knowing the 'why' behind the 'ooooh/aaahh'

I see your point. I should clarify. At some point the "light" will converge at a rate defined by the fresnel, to a point and then diverge at the same rate. At the point of convergence it will be, for lack of a better term, a pinpoint. I should not have used the term "image" as this could imply focus. Focus wasn't what I was getting at. My reasoning was geared towards how quickly the light would spread out and how close the PJ would have to be to the screen. I know the triplet is required for focus but not how it accomplishes this. I still want to know how it works exactly.

Ok, but the image is reversed is it not?

lol

Nice post supra.

'Amplifier' should read 'collector'. Would be cool if we could amplify light wouldn't it?

omg we could get something like 100,000 lumens out of a 10W bulb

Thanks, Brain.

You are correct in that a collector is the correct term. In telescopes, often the word amplifier is also used, which is where I picked up the habbit. It's kind of like in audio amplifiers where the term "RMS watts" is often used, although it, too is technically incorrect.

V&J: No, actually it will not converge on a single point. If the light coming from the LCD were truely collimated and in phase (ie a laser) then it would do so, however, there is considerable scattering effect, and as such the light would never come to a point.

To demonstrate: Take one of your fresnels (Or like I've been playing with a cheap Dollar store page magnifier) and hold it over the floor under a flourescent light fixture. If you hold it at (or near anyway) it's FL from the floor, you'll get a nice clear image of the flourescent fixture. At no distance will you get a pinpoint of light, it becomes an indistinct blur both closer and further away. If you place the lens close to the light fixture say within 1/4 of it's FL, you will never get even a focused image of the light, let alone a pinpoint. This is an example of a focused projection lens.

Now take the lens and hold it at arm's length. Assuming that your arms are longer than the FL of the lens, what you will see is the same thing that you were looking at, but inverted. The lens is not doing the focusing here, your eye is. All of the light and colour that you see in the lens is coming toward the spot where your eye is, so what would be reflected from a surface at that point would be the average colour and intensity of the whole scene Now slowly pull the lens back towards your eye. At some point a single spot will fill the entire lens and closer than that you will start to see the same thing, but no longer inverted. The point where the whole lens shows you the same spot is where it is focusing on your eye. At this point the lens is taking the light from that single point and directing it at another single point. This is the focus of the lens. If your eye were to reflect that light to an observer it would be the colour and intensity of the point htat you see. A spot next to your eye would be a spot next to the spot that you see, and so on. In other words, it is focusing the image of what you're looking at onto your face. It's not a pinpoint, and never will be.

Anything at less than it's FL will appear magnified, as seen from your viewpoint. Note that it does not matter how far away from the lens you are.

Look at the following diagram... The red, green, blue and yellow lines represent the focus from different points. On the right, you only get a focused image where the lines again come to a point. you will see that both before and after that, you never get a clear image, just a blur, and at no point ever do you get a pinpoint.

Obviously, this is not the only way that the lens can work. Closer than it's focal distance, the lens will act as a magnifier. (Dealing with convex lenses here, concave is another story altogether. )
The black line represents the focal distance of hte lens. Points before that never come to a focus, they scatter. The lens simply can't bend the light enough to bring it to a focal point, not ever. It can and does collect the light and bring it closer, though, and because of the amopunt that it does bend it another focusing lens (your eye, for example) can bring the diverse information to a focus. When doing so, the sources of the light appear further from each other, thus magnified. In the case of the projection light, it's also more concentrated, therefore brighter, which is the whole point. You can see that although the light from each point is still dispersing, it is not doing so at as great a rate as it once did.

And next, we have the example of the lens inverting. This is really an extentension of the focusing example, and is what the projection lens does. At the focal point, you can see that the image comes together, but inverted. Again, at no point does all the light come to a pinpoint. It is only bent. Although my crude drawings won't show it, the light at the edges of the lens will always be bent at the same angle, regardless of the position of the source. Naturally there will also be light at all of the points inbetween as well, bent to angles (assuming that the lens is a good one) to come to the same focal points.

There... Our little dissertation on optics has come to an end. I'm hoping that the pictures are worth 1,000 words, becasue I was at a loss for how I was going to explain all of this.

Lastly, we'll take an example of the lens at the focal point

The image at the focal point is "focused on infinity" -- This is the example where the light never comes to a focus past the lens. All light origination at a point on the focal plane becomes a parallel source. Therefore a pinpoint source becomes a column of light the size of the lens. This is what we're doing with the collimator fresnel, turning a point source of light (or as close as we can get) into a parallel projection of light. Light hitting the lens anywhere is bent to be parallel to light hitting the lens from any other point.

Important to note here: The lens does not "reject" light from a source off center to the lens, it simply deals with it in the same manner, making it parallel to that light passing through the center line. Therefore the idea that the collimator fresnel somehow rejects scattered light is simply false.

I believe that concludes today's optics lesson. Heh, I've just spent all my evening build time typing and drawing. Oh well, Tomorrow's another day.

Wow.
That's a lot of information. Thank you for taking the time to explain it so thoroughly. I have one last question though. Your diagrams show light from a non specific point. If the Collimator fresnel has pulled light mostly from a point source and the polarized LCD has further refined the light pathes, then wouldn't the majority of light striking the Field fresnel be collimated? This seems to me would cause the majority of light to converge at the Field fresnels' focal point (the reverse of the Collimator). Yeah, I'm beating it to death, but I want to understand it.

But the polarized LCD rejects off axis (non collimated light) though, right?

The pic shows a focal point of light.

In short, no.

The LCD actually scatters the collimated light from the fresnel. If it did not, then you would never be able to focus an image, only the pixels where the light happens to come through the LCD at an appropriate angle to hit the projection lens. Only light that hits that lens can be projected. True, a good part of the light through the LCD is collimated, and we're using the field fresnel to redirect it to the projection lens, but even without that lens, we COULD still project an image, but it would not be as bright. Since bright is good, we do it.

EDIT: In your image, it shows a point source of light at the focal point of hte lens. Remember that the lens is perfectly happy to do the same thing to light at a differnent point at the same distance from the lens plane.

The LCD impedes off-axis light at an angle greater than the "viewing angle" spec of hte panel. Since with good panels, this angle can be quite large, for all practical intents and purposes, it does not reject it.

I swear I read Brain say that that the LCD in effect "rejects off axis light". Now that was probably a simplified answer taken out of context referring to lumens. Your explanation certainly makes sense. Thanks again.

holy cow! I'm surrounded by NERDS!
Thanks for the in depth explanation guys.

ROFL!

'Twas a simplified answer, but to demonstrate, experiment:

Put a point light source at the focus of the collimating fresnel. Stand on the other side of the fresnel and look. For all intents and purposes you are the panel. Move the light away (up/down/side) from the focus of the fresnel and observe that the fresnel goes 'dark'.

I would try that, but I don't have my Pro fresnels yet
Oh well, I'll settle for: I'll try that soon.
In the mean time, I have some studying to do.

Actually, as long as the point light source remains at the focal distance from the fresnel, the light will simply move in the opposite direction. As long as you are diametrically opposite the lamp from the center of the fresnel, it will remain bright.

Though if it were a true point source (Which doesn't exist) exactly at the center focal point of a perfect lens, what you ought to see is the point source of light at the point which your eye is perpendicular to the lens. Well, if you want to get technical.

EDIT: The point where you see the point source of light covering the whole fresnel is where the lens has focused the light onto your eye, therefore the light is actually further from the lens than the focal point. I don't recommend you really try that with a 400W MH bulb though, as detatched retinas aren't good for you.

Yeah, the diametric opposition (Or something in that area) is something that made sense to me, and I couldn't quite mesh Brain's 'fresnels reject light' comment in with that.

Fresnels don't reject light, they just redirect it as a normal lense would.

just to make yous guyses brains fold in upon themselves, consider this for a moment...

The human eye is a fairly simple lens, so everything we "see" is actually projected on the retnia upside down and inverted. The various regions of the retina (which is actually just an "outcropping" of your brain matter) then transmit the data along various convoluded pathways (and when they are damaged, from stroke, trauma, etc you can get some really neat visual field deficits)...anyway...transmitted to various regions of the brain (some separated by *considerable* physical difference) and the squash then puts the pieces together, inverts them and turns them over, so what we see is in the correct orientation and right side up. Scary stuff.

So scary in fact, when a person has a severe stroke that affects a large portion of one or the other hemi-sphere of the brain, they WILL NOT RECOGNIZE the opposite side of their body as their own --- this is common! You can literally hold their arm up and say whos arm is this and they say "yours? i dont know." And they are otherwise cognitively intact. Vision and its interpretation is very scary stuff that you should never study late at night.

lol.. wait, what??

Correct, the lens doesn't 'reject' (excepting normal reflection which intensifies with angle, and special circumstances such as a ray intersecting the ridge of the the fresnel pitch peak) light any more than it 'amplifies' it. The lens/lcd junction performs differently though because of the angular sensitivity of the lcd. When lcd and fresnel are combined and the arc moves from the center of the focus, the picture will suffer from dimming even if the projection lens is moved to the new focus. It is correct to say the light is rejected at this point, and the light will be made manifest as heat in the lcd. It has also never been demonstrated either through logic or practice, that 'off axis' light such as that from reflection in an unpainted/flashed lamp chamber, would have any deleterious effects on the projection quality. The light 'rejection' statements arose from these arguments when someone claimed that the off axis 'rays' would emerge from the lcd and create a double image on the projection lens.